1. Field of the Invention
Embodiments of the invention generally relate to an encased electrode assembly in fluid communication with electrolyte of an electrochemical cell.
2. Description of the Related Art
Metallization of high aspect ratio 90 nm and smaller sized features is a foundational technology for future generations of integrated circuit manufacturing processes. Metallization of these features is generally accomplished via an electrochemical plating process. In a typical process scheme, a metal, such as copper, is plated onto a substrate comprising an array of integrated circuit devices with open vias and trenches. The plating process is carried out in such a manner as to fill the vias and trenches on the substrate surface with metal and to further deposit an additional amount of metal known as the overburden on the substrate. The overburden is required to enable subsequent polishing and planarization of the deposit through a process step such as chemical mechanical polishing. The total amount of metal deposited in the electrochemical plating process is typically 0.2 to 1 μm.
However, electrochemical plating (ECP) of these features presents several challenges to conventional gap fill methods and apparatuses. One such problem, for example, is that electrochemical plating processes generally require a conductive seed layer to be deposited onto the features to support the subsequent plating process. Conventionally, these seed layers have had a thickness of between about 1000 Å and about 2500 Å; however, as a result of the high aspect ratios of 90 nm features, seed layer thicknesses must be reduced to less than about 500 Å, or even below 100 Å. This reduction in the seed layer thickness increases resistivity of the substrate causing a “terminal effect,” which is an increase in the deposition thickness near the perimeter of the substrate being plated.
The terminal effect is most severe at the beginning of an electrochemical plating process, for example, within about the first 10 seconds of the electrochemical plating process, when the substrate resistivity is at the highest level. This stage is also the critical stage when the features on the substrates are being filled. The terminal effect results in a large difference in the plating rate across the substrate, leading to variations in film properties such as film composition and resistivity between the center and edge of the substrate. More importantly, a highly non-uniform plating rate across the substrate during the filling of features forces the features at either the center or edge of the wafer to fill under sub-optimal conditions, resulting in problems, such as incomplete filling and trapped voids inside the features.
Additionally, it is often desirable to modulate the plating rate at the edge of the substrate after the features have been filled and while the overburden is being deposited. For example, processes that follow electrochemical plating, such as chemical mechanical polishing, may yield better performance if the plated film is thinner at the edge than at the center of the wafer. This is because certain polishing processes are edge-slow (edge fast), so that a slightly edge-thin (edge thick) deposit profile after plating results in an optimally uniform profile after polishing.
Therefore, control of the plating rate at the edge of the substrate is desired to mitigate the terminal effect and adjust the overall plating profile. Attempts have been made modulate the plating profile at the edge of the substrate through various apparatus and methods. For example, an electrochemical polishing following the electrochemical plating process generally prefers an edge thin profile. These configurations were generally unsuccessful in controlling the terminal effect because of their lack of proximity to the perimeter of the substrate.
Additionally, conventional cells have been modified to include thief electrodes in the anolyte compartment near the edge of the substrate being plated. The thief electrodes is usually biased during plating to adjust the electric field near the vicinity of the edge of the substrate to reduce the terminal effect. However, thief electrodes introduce problems. For example, the thief electrodes usually generate defects, such as metal sludge, during operation or at rest. The use of thief electrodes also increases the breakdown rate of critical additives in the catholyte resulting in increased consumption of chemicals. Therefore, for thief electrodes to work effectively, improvement needs to be done to reduce generation of defects and consumption of chemicals.
Therefore, there exists a need for an apparatus and method for minimizing terminal effect and/or adjusting plating profile.